Printers come in various types and with various print resolutions. Regardless of the resolution, however, some applications require high-quality printing. One such application is printing one-dimensional or two-dimensional bar codes on labels or other media that can then be attached to or associated with another object, so that information about the object can be retrieved simply by scanning the associated bar code. These bar codes, especially if they are small, must be accurately printed to ensure that they can later be read by a bar code reader. Smudges, minor misalignments, discontinuous lines, and other such problems can render a printed bar code unreadable.
To help ensure that a particular printer outputs printed bar codes that meet quality standards, a verifier is sometimes coupled to the printer. A verifier commonly includes a camera that captures images of the printed barcode and then analyzes the captured images to verify that the quality of the printed barcode is acceptable. One type of verifier uses a line-scan camera, which captures images one line of pixels at a time and then assembles all the captured lines into a two-dimensional digital image that can be analyzed to determine whether the print quality of the barcode is acceptable. Other verifier embodiments use a two-dimensional image sensor to capture two-dimensional partial images of the bar code, then stitch the two-dimensional partial images together into a two-dimensional image that can be analyzed. 
In either case, whether with a line-scan camera or a 2D camera, the camera captures images in response to trigger signals. To accurately capture a complete image of the bar code, these trigger signals must be correlated to the label position or, more commonly, to the position of some element of the printer that serves as a proxy for the label position. The position of a transport roller within a printer—the element of the printer that drives the print medium, such as a label, through the printer—is one such proxy. But measuring the position of the transport roller comes with some problems.